Response of wheat canopy photosyntthesis to leaf rust: a biophysical modelling approach

Worldwide in distribution, leaf rust infection on wheat leaves can induce a large reduction of foliage, root growth, yield and grain quality by decreasing the rate of photosynthesis and increasing the rate of respiration (Bethenod et al., 2001). It is, therefore, important to understand the processes involved and to quantify the overall carbon fluxes in order to forecast disease impact and manage crops according to sustainable agricultural practices. The gas exchanges between cells are disturbed by the deterioration of membrane permeability, and gas exchanges between leaf and air are reduced by increased stomatal resistance. Upscaling the effects of disease on crops from leaf to canopy requires the study of both leaf activity and leaf ageing, the latter being responsible for leaf area reduction. A model describing the effects of disease on whole crop photosynthesis is presented. CO2 assimilation at crop scale can be calculated by a multi-layer micrometeorological model of atmospheric diffusion within and above the canopy (Bethenod et al., 2005). As no difference was found between the analytical Lagrangian approach and the classical electrical analogy, we have therefore adopted the simple generic approach of a two-layer CO2 assimilation model to examine the impact of stem rust on crops.

Leaf CO2 assimilation was modeled using a flux-gradient relationship. Each layer was characterized by three main parameters: leaf stomatal conductance (gs), leaf area index (LAI) and the ratio (k) between air CO2 concentration (Ca) and CO2 concentration (Ci) in the leaf. The net assimilation of a diseased leaf is given by the product of the net assimilation of a healthy leaf and the factor of reduction caused by the disease.

The quantification of disease effect on CO2 exchange at leaf level was simulated by applying a simple analytical approach. The model is based on Bastiaans' equation, with net leaf CO2 assimilation under disease stress relative to healthy conditions described by a power law function of the healthy leaf area percentage. Therefore the net assimilation in a given leaf layer of a diseased crop is proportional to the assimilation of the healthy foliage layer weighed by the power law function of healthy leaf percentage.

The model was tested using data from a wheat field experiment. The wheat crop was infected by Puccinia graminis, the causal agent of stem rust. Input data and parameters of the model (i.e. leaf CO2 assimilation, stomatal conductance, leaf area and internal and external CO2 concentrations) were measured in both a healthy and an infected crop. Model outputs were compared to crop transpiration and photosynthesis measured by eddy correlation and Bowen ratio systems, respectively. Crop photosynthesis was significantly reduced by leaf rust in comparison to the photosynthesis in the healthy crop. The model successfully simulated this reduction under radiation conditions favorable to the high photosynthesis values of the healthy crop. The overall comparison between measurements and model outputs indicates that the impact of rust disease on canopy photosynthesis is almost proportional to the percentage of healthy leaf area. Based on the parameterization of leaf response to disease stress, this biophysical approach provides a new way of quantifying disease effects on photosynthesis at the canopy scale with potential interest for generalization to other leaf fungal pathogens.

References

Bastiaans, L., 1990. The ratio between virtual and visual lesion size as a measure to describe reduction in leaf photosynthesis of rice due to leaf blast. Phytopathology 81, 611-615.

Bethenod, O., Huber, L., Slimi, H., 2001. Photosynthetic response of wheat to stress induced by Puccinia recondita and post-infection drought. Photosynthetica, 39, 581-590.

Bethenod O., Le Corre M., Huber L., Sache I. - 2005 - Modelling the impact of brown rust on wheat crop photosynthesis after flowering. Agricultural and Forest Meteorology, 131, 41-53.